Current measurement circuits are employed in a variety of different applications, for example, power monitoring, power consumption management, motor control, diagnostics and fault detection. Technologies that are typically used in current measurement circuits to measure current in a current-carrying conductor include sense resistors, magnetic field sensors (such as Hall-effect sensors) and current transformers.
In fault detection applications, a current measurement circuit can monitor current flow for fault conditions, e.g., by measuring current in the path from power source to load, the return path (from load to power source) or the differential between current levels in the two paths. One type of fault detection that uses differential current measurement is the ground fault interrupter (GFI). The GFI is a device designed to prevent electrical shock by detecting potentially hazardous ground faults. The GFI, when installed in a circuit, compares the amount of current in the phase (ungrounded or “hot”) conductor with the amount of current in the neutral conductor in the circuit. When a circuit is operating normally, equal current flows from the power source to the load through the phase conductor and returns through the neutral conductor. The GFI device interrupts the circuit (that is, disconnects power from the circuit) when it detects a small current difference in the phase and neutral conductors. Such a difference (typically in the range of 1 mA to 30 mA) indicates that an abnormal diversion of current from the phase conductor is occurring, e.g., as would be the case when someone touches the phase conductor. As a result, an amount of current (“ground fault current”) is returned by some path other than the intended neutral conductor.
GFI protection is required by electrical code for many household circuits, such as receptacles in bathrooms, some kitchen receptacles, some outside receptacles, and receptacles near swimming pools. To date, GFI devices have been designed to use a differential current transformer, which surrounds both the phase and the neutral conductors, to detect imbalances in the flow of current in those conductors. Such a solution tends to be bulky and expensive.